Methods are known in which switching between two colour television signals takes place as a function of the presence of a predetermined colour in one of the signals. This method is generally performed by directing a first (foreground) colour television camera onto a foreground object or objects of interest located against a monochromatic backdrop or backcloth, and replacing the portion of the foreground signal corresponding to the monochromatic backdrop by a substitute background signal produced by a second (background) colour television camera or other signal generator. Blue has proved most suitable for the monochromatic backdrop, so that this method is often called the blue screen method, although in principle any colour which is not in the foreground object of interest can be used as a switchover criterion.
If this switchover is performed with an electronic transfer switch, visually unnatural transitions often occur at the edges of the foreground object because fine details are not detected and optimization of the transitions with respect to both the luminance and chrominance information is not possible. This has led to the soft-key method in which a cross-fading process takes place between the signals of the foreground object and the substitute background.
The soft-key method may lead to colour fringing within the transition area between the signals. Another known method performs desaturation within this transition area with the foreground object signal being cross-faded into the substitute background signal (see SMPTE Journal, February 1981, pp. 107-118).
Nevertheless, this method can also lead to interference due to minor time errors of the two signals relative to one another or in the derivation of the switching or cross-fading signals. This interference becomes noticeable in the case of fine structures at the edges of the foreground object, for example on the hair or a female newsreader.